Abstract

High current multi-MeV protons and ions can be produced by irradiating thin foils with short-pulse, ultra-high intensity lasers (τ<ps, Iλ² >10¹⁸ W/cm² µm²). For many potential new applications, the high degree of beam laminarity is an important aspect, for example for table-top ion accelerators, high-resolution charged-particle radiography, or production of high energy density matter by isochoric heating. We understand the high laminarity, or low emittance, of these beams stems from the fact the acceleration process takes place on the cold rear (i.e. non-irradiated) surface of the thin foils. There, a dense relativistic electron sheath is formed by the laser-accelerated electrons that have propagated through the foil. This sheath produces an electrostatic field >10¹² V/m that ionizes the surface atoms almost instantaneously, forming a ~1 nm thick ion layer which, together with the electron sheath, resembles a virtual cathode (see Fig. 1).

© 2004 Optical Society of America